1. Field of the Invention
The present invention relates to a method for the separation of propylene oxide from reaction mixtures comprising the same resulting from the oxidation of propylene with any one of the conventional oxidizing agents, e.g., air or molecular oxygen. More especially, the invention relates to the separation of propylene oxide for such reaction mixtures by distilling the same in the presence of a neutral ester of phosphoric acid.
2. Description of the Prior Art
It is a well known phenomenon in the state of the art that during the oxidation, or perhaps more correctly the epoxidation of propylene, a variety of by-products are simultaneously formed, certain of which markedly interfere with the ultimate yields to be attained by reason of their propensity of reaction with the desired propylene oxide end product. Thus, it is known, for example, that formic acid, and to a lesser extent, acetic acid, rapidly react with the epoxides. Compare H. W. Gibson, Chem. Rev., 5, 675 (1969); W. J. Hickinbottom et al., J. Chem. Soc., 4200 (1954). Such reaction is more pronounced in the case of propylene oxide, especially in the presence of small amounts of water, giving rise to the production of propylene glycol and certain esters thereof, as well as certain higher molecular weight products, such as the polypropylene glycols which usually are to some extent esterified. This type of reaction is especially pronounced during the separation, by simple distillation procedures, of fractions rich in the desired propylene oxide resulting from the liquid or condensed reaction mixtures emanating from a typical propylene oxidation. The ultimate result of course is a considerable loss of desired, initially produced epoxide.
And steps have been taken in the art to obviate the aforesaid difficulties by conducting the distillation of the propylene oxide admixture in the presence of certain additives which tend to minimize or oppose the reaction between the desired epoxide compound and the noted organic acids. According to U.S. Pat. No. 3,715,284, for example, the propylene oxide is distilled upon addition to the crude oxidation product or admixture, of acetone, optionally admixed with methanol. Such additives are employed to effect a physical separation, in the distillation column, of the propylene oxide from the noted acid and thus render impossible the competing, undesirable reactions. It has also been proposed to add an ester, for example, ethyl acetate, to the reaction medium to be treated. See French Pat. No. 2,018,290. However, this latter type of process necessitates the utilization of certain additives, the boiling points of which must be between the boiling point of propylene oxide and those of the noted acids. Consequently, in order to recycle any such additive, it of course is necessary to distill it in its entirety. This latter type of operation too is required for recovery of the acids. In sum, the end result is a significant energy requirement.